Onto what root stock should I graft a Ginkgo biloba scion?
According to The Complete Book of Plant Propagation (Taunton Press, 1997, Jim Arbury et al.), Ginkgo biloba can be propagated without grafting, by taking semi-ripe cuttings in midsummer and dusting them with rooting hormone and potting them up in a mixture of half peat, half sand/vermiculite. Cuttings should root by spring if kept moist, and need to be planted out once they have roots.
If you wish to graft it, you need a Ginkgo biloba rootstock, which you could grow from seed if you have access to a female ginkgo tree (they are hard to find), and (according to the American Horticultural Society Plant Propagation manual) you can use a whip-and-tongue or spliced side veneer graft done in late winter. The AHS manual also recommends taking softwood cuttings in late spring or early summer for ginkgo.
There is also helpful information from The Ginkgo Pages and Plants for a Future Database, which says that softwood cuttings are taken in spring, semi-ripe cuttings are taken in July and August, and hardwood cuttings are taken in December, and all are kept in a frame.